EMI filter employing a capacitor and an inductor tank circuit having optimum component values

ABSTRACT

A bandstop filter having optimum component values is provided for a lead of an active implantable medical device (AIMD). The bandstop filter includes a capacitor in parallel with an inductor. The parallel capacitor and inductor are placed in series with the implantable lead of the AIMD, wherein values of capacitance and inductance are selected such that the bandstop filter is resonant at a selected frequency. The Q of the inductor may be relatively maximized and the Q of the capacitor may be relatively minimized to reduce the overall Q of the bandstop filter to attenuate current flow through the implantable lead along a range of selected frequencies.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. patent applicationSer. No. 11/423,073, filed on Jun. 8, 2006, now U.S. Pat. No. 8,244,370.

BACKGROUND OF THE INVENTION

This invention relates generally to novel EMI tank filter assemblies,particularly of the type used in connection with active implantablemedical devices (AIMDs) such as cardiac pacemakers, cardioverterdefibrillators, neurostimulators, and the like, which decoupleimplantable leads and/or electronic components of the implantablemedical device from undesirable electromagnetic interference (EMI)signals at a selected frequency or frequencies, such as the RF pulsedfields of Magnetic Resonance Imaging (MRI) equipment.

Compatibility of cardiac pacemakers, implantable defibrillators andother types of active implantable medical devices with magneticresonance imaging (MRI) and other types of hospital diagnostic equipmenthas become a major issue. If one goes to the websites of the majorcardiac pacemaker manufacturers in the United States, which include St.Jude Medical, Medtronic and Boston Scientific (formerly Guidant), onewill see that the use of MRI is generally contra-indicated withpacemakers and implantable defibrillators. See also: (1) Safety Aspectsof Cardiac Pacemakers in Magnetic Resonance Imaging”, a dissertationsubmitted to the Swiss Federal Institute of Technology Zurich presentedby Roger Christoph Luchinger, Zurich 2002; (2) “1. Dielectric Propertiesof Biological Tissues: Literature Survey”, by C. Gabriel, S. Gabriel andE. Cortout; (3) “II. Dielectric Properties of Biological Tissues:Measurements and the Frequency Range 0 Hz to 20 GHz”, by S. Gabriel, R.W. Lau and C. Gabriel; (4) “III. Dielectric Properties of BiologicalTissues: Parametric Models for the Dielectric Spectrum of Tissues”, byS. Gabriel, R. W. Lau and C. Gabriel; and (5) “Advanced EngineeringElectromagnetics, C. A. Balanis, Wiley, 1989; (6) Systems and Methodsfor Magnetic-Resonance-Guided Interventional Procedures, PatentApplication Publication US 2003/0050557, Susil and Halperin et. al,published Mar. 13, 2003; (7) Multifunctional Interventional Devices forMRI: A Combined Electrophysiology/MRI Catheter, by, Robert C. Susil,Henry R. Halperin, Christopher J. Yeung, Albert C. Lardo and ErginAtalar, MRI in Medicine, 2002; and (8) Multifunctional InterventionalDevices for Use in MRI, U.S. Patent Application Ser. No. 60/283,725,filed Apr. 13, 2001.

The contents of the foregoing are all incorporated herein by reference.

However, an extensive review of the literature indicates that MRI isindeed often used with pacemaker, neurostimulator and other activeimplantable medical device (AIMD) patients. The safety and feasibilityof MRI in patients with cardiac pacemakers is an issue of gainingsignificance. The effects of MRI on patients' pacemaker systems haveonly been analyzed retrospectively in some case reports. There are anumber of papers that indicate that MRI on new generation pacemakers canbe conducted up to 0.5 Tesla (T). MRI is one of medicine's most valuablediagnostic tools. MRI is, of course, extensively used for imaging, butis also used for interventional medicine (surgery). In addition, MRI isused in real time to guide ablation catheters, neurostimulator tips,deep brain probes and the like. An absolute contra-indication forpacemaker patients means that pacemaker and ICD wearers are excludedfrom MRI. This is particularly true of scans of the thorax and abdominalareas. Because of MRI's incredible value as a diagnostic tool forimaging organs and other body tissues, many physicians simply take therisk and go ahead and perform MRI on a pacemaker patient. The literatureindicates a number of precautions that physicians should take in thiscase, including limiting the power of the MRI RF Pulsed field (SpecificAbsorption Rate—SAR), programming the pacemaker to fixed or asynchronouspacing mode, and then careful reprogramming and evaluation of thepacemaker and patient after the procedure is complete. There have beenreports of latent problems with cardiac pacemakers or other AIMDs afteran MRI procedure sometimes occurring many days later. Moreover, thereare a number of recent papers that indicate that the SAR level is notentirely predictive of the heating that would be found in implantedleads or devices. For example, for magnetic resonance imaging devicesoperating at the same magnetic field strength and also at the same SARlevel, considerable variations have been found relative to heating ofimplanted leads. It is speculated that SAR level alone is not a goodpredictor of whether or not an implanted device or its associated leadsystem will overheat.

There are three types of electromagnetic fields used in an MRI unit. Thefirst type is the main static magnetic field designated B₀ which is usedto align protons in body tissue. The field strength varies from 0.5 to3.0 Tesla in most of the currently available MRI units in clinical use.Some of the newer MRI system fields can go as high as 4 to 5 Tesla. Atthe recent International Society for Magnetic Resonance in Medicine(ISMRM), which was held on 5 and 6 Nov. 2005, it was reported thatcertain research systems are going up as high as 11.7 Tesla and will beready sometime in 2006. This is over 100,000 times the magnetic fieldstrength of the earth. A static magnetic field can induce powerfulmechanical forces and torque on any magnetic materials implanted withinthe patient. This would include certain components within the cardiacpacemaker itself and or lead systems. It is not likely (other thansudden system shut down) that the static MRI magnetic field can inducecurrents into the pacemaker lead system and hence into the pacemakeritself. It is a basic principle of physics that a magnetic field musteither be time-varying as it cuts across the conductor, or the conductoritself must move within the magnetic field for currents to be induced.

The second type of field produced by magnetic resonance imaging is thepulsed RF field which is generated by the body coil or head coil. Thisis used to change the energy state of the protons and illicit MRIsignals from tissue. The RF field is homogeneous in the central regionand has two main components: (1) the magnetic field is circularlypolarized in the actual plane; and (2) the electric field is related tothe magnetic field by Maxwell's equations. In general, the RF field isswitched on and off during measurements and usually has a frequency of21 MHz to 64 MHz to 128 MHz depending upon the static magnetic fieldstrength. The frequency of the RF pulse varies with the field strengthof the main static field where: RF PULSED FREQUENCY in MHz=(42.56)(STATIC FIELD STRENGTH IN TESLA).

The third type of electromagnetic field is the time-varying magneticgradient fields designated B₁ which are used for spatial localization.These change their strength along different orientations and operatingfrequencies on the order of 1 kHz. The vectors of the magnetic fieldgradients in the X, Y and Z directions are produced by three sets oforthogonally positioned coils and are switched on only during themeasurements. In some cases, the gradient field has been shown toelevate natural heart rhythms (heart beat). This is not completelyunderstood, but it is a repeatable phenomenon. The gradient field is notconsidered by many researchers to create any other adverse effects.

It is instructive to note how voltages and EMI are induced into animplanted lead system. At very low frequency (VLF), voltages are inducedat the input to the cardiac pacemaker as currents circulate throughoutthe patient's body and create voltage drops. Because of the vectordisplacement between the pacemaker housing and, for example, the TIPelectrode, voltage drop across the resistance of body tissues may besensed due to Ohms Law and the circulating current of the RF signal. Athigher frequencies, the implanted lead systems actually act as antennaswhere currents are induced along their length. These antennas are notvery efficient due to the damping effects of body tissue; however, thiscan often be offset by extremely high power fields (such as MRI pulsedfields) and/or body resonances. At very high frequencies (such ascellular telephone frequencies), EMI signals are induced only into thefirst area of the lead system (for example, at the header block of acardiac pacemaker). This has to do with the wavelength of the signalsinvolved and where they couple efficiently into the system.

Magnetic field coupling into an implanted lead system is based on loopareas. For example, in a cardiac pacemaker, there is a loop formed bythe lead as it comes from the cardiac pacemaker housing to its distalTIP, for example, located in the right ventricle. The return path isthrough body fluid and tissue generally straight from the TIP electrodein the right ventricle back up to the pacemaker case or housing. Thisforms an enclosed area which can be measured from patient X-rays insquare centimeters. The average loop area is 200 to 225 squarecentimeters. This is an average and is subject to great statisticalvariation. For example, in a large adult patient with an abdominalimplant, the implanted loop area is much larger (greater than 450 squarecentimeters).

Relating now to the specific case of MRI, the magnetic gradient fieldswould be induced through enclosed loop areas. However, the pulsed RFfields, which are generated by the body coil, would be primarily inducedinto the lead system by antenna action.

There are a number of potential problems with MRI, including:

(1) Closure of the pacemaker reed switch. A pacemaker reed switch, whichcan also be a Hall Effect device, is designed to detect a permanentmagnet held close to the patient's chest. This magnet placement allows aphysician or even the patient to put the implantable medical device intowhat is known as the “magnet mode response.” The “magnet mode response”varies from one manufacturer to another, however, in general, this putsthe pacemaker into a fixed rate or asynchronous pacing mode. This isnormally done for short times and is very useful for diagnostic andclinical purposes. However, in some cases, when a pacemaker is broughtinto the bore or close to the MRI scanner, the MRI static field can makethe pacemaker's internal reed switch close, which puts the pacemakerinto a fixed rate or asynchronous pacing mode. Worse yet, the reedswitch may bounce or oscillate. Asynchronous pacing may compete with thepatient's underlying cardiac rhythm. This is one reason why patientshave generally been advised not to undergo MRI. Fixed rate orasynchronous pacing for most patients is not an issue. However, inpatients with unstable conditions, such as myocardial ischemia, there isa substantial risk for ventricular fibrillation during asynchronouspacing. In most modern pacemakers the magnetic reed switch (or HallEffect device) function is programmable. If the magnetic reed switchresponse is switched off, then synchronous pacing is still possible evenin strong magnetic fields. The possibility to open and re-close the reedswitch in the main magnetic field by the gradient field cannot beexcluded. However, it is generally felt that the reed switch will remainclosed due to the powerful static magnetic field. It is theoreticallypossible for certain reed switch orientations at the gradient field tobe capable of repeatedly closing and re-opening the reed switch.

(2) Reed switch damage. Direct damage to the reed switch istheoretically possible, but has not been reported in any of the knownliterature. In an article written by Roger Christoph Luchinger ofZurich, he reports on testing in which reed switches were exposed to thestatic magnetic field of MRI equipment. After extended exposure to thesestatic magnetic fields, the reed switches functioned normally at closeto the same field strength as before the test.

(3) Pacemaker displacement. Some parts of pacemakers, such as thebatteries and reed switch, contain ferrous magnetic materials and arethus subject to mechanical forces during MRI. Pacemaker displacement mayoccur in response to magnetic force or magnetic torque. There areseveral recent reports on modern pacemakers and ICDs that force andtorque are not of concern for MRI systems up to 3 Tesla.

(4) Radio frequency field. At the frequencies of interest in MRI, RFenergy can be absorbed and converted to heat. The power deposited by RFpulses during MRI is complex and is dependent upon the power (SpecificAbsorption Rate (SAR) Level) and duration of the RF pulse, thetransmitted frequency, the number of RF pulses applied per unit time,and the type of configuration of the RF transmitter coil used. Theamount of heating also depends upon the volume of tissue imaged, theelectrical resistivity of tissue and the configuration of the anatomicalregion imaged. There are also a number of other variables that depend onthe placement in the human body of the AIMD and its associated lead(s).For example, it will make a difference how much current is induced intoa pacemaker lead system as to whether it is a left or right pectoralimplant. In addition, the routing of the lead and the lead length arealso very critical as to the amount of induced current and heating thatwould occur. Also, distal TIP design is very important as the distal TIPitself can act as its own antenna wherein eddy currents can createheating. The cause of heating in an MRI environment is twofold:

-   -   (a) RF field coupling to the lead can occur which induces        significant local heating; and    -   (b) currents induced between the distal TIP and tissue during        MRI RF pulse transmission sequences can cause local Ohms Law        heating in tissue next to the distal TIP electrode of the        implanted lead. The RF field of an MRI scanner can produce        enough energy to induce lead currents sufficient to destroy some        of the adjacent myocardial tissue. Tissue ablation has also been        observed. The effects of this heating are not readily detectable        by monitoring during the MRI. Indications that heating has        occurred would include an increase in pacing threshold, venous        ablation, Larynx or e ablation, myocardial perforation and lead        penetration, or even arrhythmias caused by scar tissue. Such        long term heating effects of MRI have not been well studied yet        for all types of AIMD lead geometries. There can also be        localized heating problems associated with various types of        electrodes in addition to TIP electrodes. This includes RING        electrodes or PAD electrodes. RING electrodes are commonly used        with a wide variety of implanted devices including cardiac        pacemakers, and neurostimulators, and the like. PAD electrodes        are very common in neurostimulator applications. For example,        spinal cord stimulators or deep brain stimulators can include a        plurality of PAD electrodes to make contact with nerve tissue. A        good example of this also occurs in a cochlear implant. In a        typical cochlear implant there would be sixteen RING electrodes        that the physician places by pushing the electrode up into the        cochlea. Several of these RING electrodes make contact with        auditory nerves.    -   (5) Alterations of pacing rate due to the applied radio        frequency field. It has been observed that the RF field may        induce undesirable fast pacing (QRS complex) rates. There are        various mechanisms which have been proposed to explain rapid        pacing: direct tissue stimulation, interference with pacemaker        electronics or pacemaker reprogramming (or reset). In all of        these cases, it is very desirable to raise the lead system        impedance (at the MRI RF pulsed frequency) to make an EMI filter        feedthrough capacitor more effective and thereby provide a        higher degree of protection to AIMD electronics. This will make        alterations in pacemaker pacing rate and/or pacemaker        reprogramming much more unlikely.    -   (6) Time-varying magnetic gradient fields. The contribution of        the time-varying gradient to the total strength of the MRI        magnetic field is negligible, however, pacemaker systems could        be affected because these fields are rapidly applied and        removed. The time rate of change of the magnetic field is        directly related to how much electromagnetic force and hence        current can be induced into a lead system. Luchinger reports        that even using today's gradient systems with a time-varying        field up to 50 Tesla per second, the induced currents are likely        to stay below the biological thresholds for cardiac        fibrillation. A theoretical upper limit for the induced voltage        by the time-varying magnetic gradient field is 20 volts. Such a        voltage during more than 0.1 milliseconds could be enough energy        to directly pace the heart.    -   (7) Heating. Currents induced by time-varying magnetic gradient        fields may lead to local heating. Researchers feel that the        calculated heating effect of the gradient field is much less as        compared to that caused by the RF field and therefore for the        purposes herein may be neglected.

There are additional problems possible with implantable cardioverterdefibrillators (ICDs). ICDs use different and larger batteries whichcould cause higher magnetic forces. The programmable sensitivity in ICDsis normally much higher (more sensitive) than it is for pacemakers,therefore, ICDs may falsely detect a ventricular tachyarrhythmia andinappropriately deliver therapy. In this case, therapy might includeanti-tachycardia pacing, cardio version or defibrillation (high voltageshock) therapies. MRI magnetic fields may prevent detection of adangerous ventricular arrhythmia or fibrillation. There can also beheating problems of ICD leads which are expected to be comparable tothose of pacemaker leads. Ablation of vascular walls is another concern.Fortunately, ICDs have a sort of built-in fail-safe mechanism. That is,during an MRI procedure, if they inadvertently sense the MRI fields as adangerous ventricular arrhythmia, the ICD will attempt to charge up anddeliver a high voltage shock. However, there is a transformer containedwithin the ICD that is necessary to function in order to charge up thehigh-energy storage capacitor contained within the ICD. In the presenceof the main static field of the MRI the core of this transformer tendsto saturate thereby preventing the high voltage capacitor from chargingup. This makes it highly unlikely that an ICD patient undergoing an MRIwould receive an inappropriate high voltage shock therapy. While ICDscannot charge during MRI due to the saturation of their ferro-magnetictransformers, the battery will be effectively shorted and lose life.This is a highly undesirable condition.

In summary, there are a number of studies that have shown that MRIpatients with active implantable medical devices, such as cardiacpacemakers, can be at risk for potential hazardous effects. However,there are a number of reports in the literature that MRI can be safe forimaging of pacemaker patients when a number of precautions are taken(only when an MRI is thought to be an absolute diagnostic necessity).While these anecdotal reports are of interest, however, they arecertainly not scientifically convincing that all MRI can be safe. Aspreviously mentioned, just variations in the pacemaker lead length cansignificantly effect how much heat is generated. From the layman's pointof view, this can be easily explained by observing the typical length ofthe antenna on a cellular telephone compared to the vertical rod antennamore common on older automobiles. The relatively short antenna on thecell phone is designed to efficiently couple with the very highfrequency wavelengths (approximately 950 MHz) of cellular telephonesignals. In a typical AM and FM radio in an automobile, these wavelengthsignals would not efficiently couple to the relatively short antenna ofa cell phone. This is why the antenna on the automobile is relativelylonger. An analogous situation exists with an AIMD patient in an MRIsystem. If one assumes, for example, a 3.0 Tesla MRI system, which wouldhave an RF pulsed frequency of 128 MHz, there are certain implanted leadlengths that would couple efficiently as fractions of the 128 MHzwavelength. It is typical that a hospital will maintain an inventory ofvarious leads and that the implanting physician will make a selectiondepending on the size of the patient, implant location and otherfactors. Accordingly, the implanted or effective lead length can varyconsiderably. There are certain implanted lead lengths that just do notcouple efficiently with the MRI frequency and there are others thatwould couple very efficiently and thereby produce the worst case forheating.

The effect of an MRI system on the function of pacemakers, ICDs andneurostimulators depends on various factors, including the strength ofthe static magnetic field, the pulse sequence (gradient and RF fieldused), the anatomic region being imaged, and many other factors. Furthercomplicating this is the fact that each patient's condition andphysiology is different and each manufacturer's pacemaker and ICDdesigns also are designed and behave differently. Most experts stillconclude that MRI for the pacemaker patient should not be consideredsafe. Paradoxically, this also does not mean that the patient should notreceive MRI. The physician must make an evaluation given the pacemakerpatient's condition and weigh the potential risks of MRI against thebenefits of this powerful diagnostic tool. As MRI technology progresses,including higher field gradient changes over time applied to thinnertissue slices at more rapid imagery, the situation will continue toevolve and become more complex. An example of this paradox is apacemaker patient who is suspected to have a cancer of the lung. RFablation treatment of such a tumor may require stereotactic imaging onlymade possible through real time fine focus MRI. With the patient's lifeliterally at risk, the physician with patient informed consent may makethe decision to perform MRI in spite of all of the previously describedattendant risks to the pacemaker system.

Insulin drug pump systems do not seem to be of a major current concerndue to the fact that they have no significant antenna components (suchas implanted leads). However, some implantable pumps work onmagneto-peristaltic systems, and must be deactivated prior to MRI. Thereare newer (unreleased) systems that would be based on solenoid systemswhich will have similar problems.

It is clear that MRI will continue to be used in patients with activeimplantable medical devices. Accordingly, there is a need for AIMDsystem and/or circuit protection devices which will improve the immunityof active implantable medical device systems to diagnostic proceduressuch as MRI.

As one can see, many of the undesirable effects in an implanted leadsystem from MRI and other medical diagnostic procedures are related toundesirable induced currents in the lead system and/or its distal TIP(or RING). This can lead to overheating either in the lead or at thebody tissue at the distal TIP. For a pacemaker application, thesecurrents can also directly stimulate the heart into sometimes dangerousarrhythmias.

Accordingly, there is a need for a novel resonant bandstop filterassembly which can be placed at various locations along the activeimplantable medical device lead system, which also prevents current fromcirculating at selected frequencies of the medical therapeutic device.Preferably, such novel tank filters would be designed to resonate at 64MHz for use in an MRI system operating at 1.5 Tesla (or 128 MHz for a 3Tesla system). The present invention fulfills these needs and providesother related advantages.

SUMMARY OF THE INVENTION

The present invention comprises resonant tank circuits/bandstop filtersto be placed at one or more locations along the active implantablemedical device (AIMD) lead system, including its distal Tip. Thesebandstop filters prevent current from circulating at selectedfrequencies of the medical therapeutic device. For example, for an MRIsystem operating at 1.5 Tesla, the pulse RF frequency is 64 MHz. Thenovel bandstop filters of the present invention can be designed toresonate at 64 MHz and thus create an open circuit in the implanted leadsystem at that selected frequency. For example, the bandstop filter ofthe present invention, when placed at the distal TIP, will preventcurrents from flowing through the distal TIP, prevent currents fromflowing in the implanted leads and also prevent currents from flowinginto body tissue. It will be obvious to those skilled in the art thatall of the embodiments described herein are equally applicable to a widerange of other active implantable medical devices, including deep brainstimulators, spinal cord stimulators, cochlear implants, ventricularassist devices, artificial hearts, drug pumps, and the like. The presentinvention fulfills all of the needs regarding reduction or eliminationof undesirable currents and associated heating in implanted leadsystems.

Electrically engineering a capacitor in parallel with an inductor isknown as a tank filter. It is also well known that when the tank filteris at its resonant frequency, it will present a very high impedance.This is a basic principle of all radio receivers. In fact, multiple tankfilters are often used to improve the selectivity of a radio receiver.One can adjust the resonant frequency of the tank circuit by eitheradjusting the capacitor value or the inductor value or both. Sincemedical diagnostic equipment which is capable of producing very largefields operates at discrete frequencies, this is an ideal situation fora specific tank or bandstop filter. Bandstop filters are more efficientfor eliminating one single frequency than broadband filters. Because thebandstop filter is targeted at this one frequency or range offrequencies, it can be much smaller and volumetrically efficientsuitable for incorporation into an implantable medical device. Inaddition, the way MRI RF pulse fields couple with lead systems, variousloops and associated loop currents result along various sections of theimplanted lead. For example, at the distal TIP of a cardiac pacemaker,direct electromagnetic forces (EMF) can be produced which result incurrent loops through the distal TIP and into the associated myocardialtissue. This current system is largely decoupled from the currents thatare induced near the active implantable medical device, for example,near the cardiac pacemaker. There the MRI may set up a separate loopwith its associated currents. Accordingly, one or more bandstop filtersmay be required to completely control all of the various induced EMI andassociated currents in a lead system.

The present invention which resides in bandstop filters is also designedto work in concert with the EMI filter which is typically used at thepoint of lead ingress and egress of the active implantable medicaldevice. For example, see U.S. Pat. Nos. 5,333,095 and 6,999,818; andU.S. Patent Publication Nos. US-2005-0197677-A1 and US-2007-0083244-A1;the contents of all being incorporated herein by reference. All of thesepatent documents describe novel inductor capacitor combinations for lowpass EMI filter circuits. It is of particular interest that byincreasing the number of circuit elements, one can reduce the overallcapacitance value which is at the input to the implantable medicaldevice. It is important to reduce the capacitance value to raise theinput impedance of the active implantable medical device such that thisalso reduces the amount of current that would flow in lead systemsassociated with medical procedures such as MRI. Accordingly, it is afeature of the present invention that the novel bandstop filters aredesigned to be used in concert with the structures described in theabove mentioned three patent applications.

In one embodiment, the invention provides a medical therapeutic devicecomprising an active implantable medical device (AIMD), an implantablelead extending from the AIMD to a distal TIP thereof, and a bandstopfilter associated with the implantable lead for attenuating current flowthrough the lead at a selected frequency.

The AIMD may comprise cochlear implants, piezoelectric sound bridgetransducers, neurostimulators, brain stimulators, cardiac pacemakers,ventricular assist devices, artificial hearts, drug pumps, bone growthstimulators, bone fusion stimulators, urinary incontinence devices, painrelief spinal cord stimulators, anti-tremor stimulators, gastricstimulators, implantable cardioverter defibrillators, pH probes,congestive heart failure devices, neuromodulators, cardiovascularstents, orthopedic implants, and the like.

The bandstop filter itself comprises a capacitor (and its resistance oran added resistance) in parallel with an inductor (and its parasiticresistance), said parallel capacitor and inductor combination beingplaced in series with the medical device implantable lead(s) wherein thevalues of capacitance and inductance have been selected so as to fallwithin the range of 0.1-20,000 picofarads (preferably 1-100 picofarads)and 1-4000 nanohenries (preferably 100-1000 nanohenries), respectively,such that the bandstop filter is resonant at a selected frequency (suchas the MRI pulsed frequency).

In the preferred embodiment, the overall Q factor of the bandstop filteris selected to balance impedance at the selected frequency versusfrequency band width characteristics. More specifically, the Q of theinductor is relatively maximized and the Q of the capacitor isrelatively minimized to reduce the overall Q of the bandstop filter. TheQ of the inductor is relatively maximized by minimizing the parasiticresistive loss in the inductor, and the Q of the capacitor is relativelyminimized by raising its equivalent series resistance (ESR) of thecapacitor (or by adding resistance or a resistive element in series withthe capacitor element of the bank stop tank filter). This reduces theoverall Q of the bandstop filter in order to broaden its 3 dB points andthereby attenuate current flow through the lead along a range ofselected frequencies. In AIMD or external medical device applications,the range of selected frequencies includes a plurality of MRI pulsedfrequencies.

The equivalent series resistance of the capacitor is raised by any ofthe following: reducing thickness of electrode plates in the capacitor;using higher resistivity capacitor electrode materials, providingapertures, gaps, slits or spokes in the electrode plates of thecapacitor; providing separate discrete resistors in series with thecapacitor; utilizing resistive electrical attachment materials to thecapacitor; or utilizing capacitor dielectric materials that have highdielectric loss tangents at the selected frequency. Methods of usinghigher resistivity capacitor electrode materials include, for example,using platinum instead of silver electrodes. Platinum has a highervolume resistivity as compared to pure silver. Another way of reducingcapacitor electrode plate resistivity is to add ceramic powders to theelectrode ink before it is silk screened down and fired. After firing,this has the effect of separating the conductive electrode portions byinsulative dielectric areas which increases the overall resistivity ofthe electrode plate.

As defined herein, raising the capacitor ESR includes any or all of theabove described methods of adding resistance in series with thecapacitive element of the bandstop filter. It should be noted thatdeliberately raising the capacitor ESR runs counter toconventional/prior art capacitor technologies. In fact, capacitormanufacturers generally strive to build capacitors with as low an ESR aspossible. This is to minimize energy loss, etc. It is a feature of thepresent invention that capacitor Q is raised in a controlled manner inthe tank filter circuit in order to adjust its Q and adjust the bandstopfrequency width in the range of MRI pulsed frequencies.

Preferably, the bandstop filter is disposed adjacent to the distal tipof the lead and is integrated into a TIP electrode. It may also beintegrated into one or more RING electrodes.

The present invention also provides a novel process for attenuatingcurrent flow through an implantable lead for an active implantablemedical device at a selected frequency, comprising the steps of:selecting a capacitor which is resonant at the selected frequency;selecting an inductor which is resonant at the selected frequency; usingthe capacitor and the inductor to form a tank filter circuit; andplacing the tank filter circuit in series with the lead.

The overall Q of the tank filter circuit may be reduced by increasingthe Q of the inductor and reducing the Q of the capacitor. In thisregard, minimizing resistive loss in the inductor maximizes the Q of theinductor, and raising the equivalent series resistance of the capacitorminimizes the Q of the capacitor.

The net effect is to reduce the overall Q of the tank filter circuitwhich widens the bandstop width to attenuate current flow through thelead along a range of selected frequencies. As discussed herein, therange of selected frequencies may include a plurality of MRI pulsefrequencies.

Other features and advantages of the present invention will becomeapparent from the following more detailed description, taken inconjunction with the accompanying drawings which illustrate, by way ofexample, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate the invention. In such drawings:

FIG. 1 is a wire-formed diagram of a generic human body showing a numberof active implantable medical devices (AIMDs);

FIG. 2 is a perspective and somewhat schematic view of a prior artactive implantable medical device (AIMD) including a lead directed tothe heart of a patient;

FIG. 3 is an enlarged sectional view taken generally along the line 3-3of FIG. 2;

FIG. 4 is a view taken generally along the line 4-4 of FIG. 3;

FIG. 5 is a perspective/isometric view of a prior art rectangularquadpolar feedthrough capacitor of the type shown in FIGS. 3 and 4;

FIG. 6 is sectional view taken generally along the line 6-6 of FIG. 5;

FIG. 7 is a sectional view taken generally along the line 7-7 of FIG. 5.

FIG. 8 is a diagram of a unipolar active implantable medical device;

FIG. 9 is a diagram similar to FIG. 8, illustrating a bipolar AIMDsystem;

FIG. 10 is a diagram similar to FIGS. 8 and 9, illustrating a biopolarlead system with a distal TIP and RING, typically used in a cardiacpacemaker;

FIG. 11 is a schematic diagram showing a parallel combination of aninductor L and a capacitor C placed in series with the lead systems ofFIGS. 8-10;

FIG. 12 is a chart illustrating calculation of frequency of resonancefor the parallel tank circuit of FIG. 11;

FIG. 13 is a graph showing impedance versus frequency for the paralleltank bandstop circuit of FIG. 11;

FIG. 14 is an equation for the impedance of an inductor in parallel witha capacitor;

FIG. 15 is a chart illustrating reactance equations for the inductor andthe capacitor of the parallel tank circuit of FIG. 11;

FIG. 16 is a schematic diagram illustrating the parallel tank circuit ofFIG. 11, except in this case the inductor and the capacitor have seriesresistive losses;

FIG. 17 is a diagram similar to FIG. 8, illustrating the tankcircuit/bandstop filter added near a distal electrode;

FIG. 18 is a schematic representation of the novel bandstop tank filterof the present invention, using switches to illustrate its function atvarious frequencies;

FIG. 19 is a schematic diagram similar to FIG. 18, illustrating the lowfrequency model of the bandstop filter;

FIG. 20 is a schematic diagram similar to FIGS. 18 and 19, illustratingthe model of the bandstop filter of the present invention at itsresonant frequency;

FIG. 21 is a schematic diagram similar to FIGS. 18-20, illustrating amodel of the bandstop filter at high frequencies well above the resonantfrequency;

FIG. 22 is a decision tree block diagram illustrating a process fordesigning the bandstop filters of the present invention;

FIG. 23 is graph of insertion loss versus frequency for bandstop filtershaving high Q inductors and differing quality “Q” factors;

FIG. 24 is a tracing of an exemplary patient x-ray showing an implantedpacemaker and cardioverter defibrillator and corresponding lead system;

FIG. 25 is a line drawings of an exemplary patent cardiac x-ray of abi-ventricular lead system;

FIG. 26 illustrates a bipolar cardiac pacemaker lead showing the distalTIP and the distal RING electrodes; and

FIG. 27 is an enlarged, fragmented schematic illustration of the areaillustrated by the line 27-27 in FIG. 26.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates of various types of active implantable medicaldevices 100 that are currently in use. FIG. 1 is a wire formed diagramof a generic human body showing a number of implanted medical devices.100A is a family of implantable hearing devices which can include thegroup of cochlear implants, piezoelectric sound bridge transducers andthe like. 100B includes an entire variety of neurostimulators and brainstimulators. Neurostimulators are used to stimulate the Vagus nerve, forexample, to treat epilepsy, obesity and depression. Brain stimulatorsare similar to a pacemaker-like device and include electrodes implanteddeep into the brain for sensing the onset of the seizure and alsoproviding electrical stimulation to brain tissue to prevent the seizurefrom actually happening. 100C shows a cardiac pacemaker which iswell-known in the art. 100D includes the family of left ventricularassist devices (LVAD's), and artificial hearts, including the recentlyintroduced artificial heart known as the Abiocor. 100E includes anentire family of drug pumps which can be used for dispensing of insulin,chemotherapy drugs, pain medications and the like. Insulin pumps areevolving from passive devices to ones that have sensors and closed loopsystems. That is, real time monitoring of blood sugar levels will occur.These devices tend to be more sensitive to EMI than passive pumps thathave no sense circuitry or externally implanted leads. 100F includes avariety of implantable bone growth stimulators for rapid healing offractures. 100G includes urinary incontinence devices. 100H includes thefamily of pain relief spinal cord stimulators and anti-tremorstimulators. 100H also includes an entire family of other types ofneurostimulators used to block pain. 100I includes a family ofimplantable cardioverter defibrillators (ICD) devices and also includesthe family of congestive heart failure devices (CHF). This is also knownin the art as cardio resynchronization therapy devices, otherwise knownas CRT devices.

Referring now to FIG. 2, a prior art active implantable medical device(AIMD) 100 is illustrated. In general, the AIMD 100 could, for example,be a cardiac pacemaker 100C which is enclosed by a titanium housing 102as indicated. The titanium housing is hermetically sealed, however thereis a point where leads 104 must ingress and egress the hermetic seal.This is accomplished by providing a hermetic terminal assembly 106.Hermetic terminal assemblies are well known and generally consist of aferrule 108 which is laser welded to the titanium housing 102 of theAIMD 100. The hermetic terminal assembly 106 with its associated EMIfilter is better shown in FIG. 3. Referring once again to FIG. 2, fourleads are shown consisting of lead pair 104 a and 104 b and lead pair104 c and 104 d. This is typical of what's known as a dual chamberbipolar cardiac pacemaker.

The ISI connectors 110 that are designed to plug into the header block112 are low voltage (pacemaker) connectors covered by an ANSI/AAMIstandard IS-1. Higher voltage devices, such as implantable cardioverterdefibrillators, are covered by a standard known as the ANSI/AAMI DF-1.There is a new standard under development which will integrate both highvoltage and low voltage connectors into a new miniature connector seriesknown as the IS-4 series. These connectors are typically routed in apacemaker application down into the right ventricle and right atrium ofthe heart 114. There are also new generation devices that have beenintroduced to the market that couple leads to the outside of the leftventricle. These are known as biventricular devices and are veryeffective in cardiac resynchronization therapy (CRT) and treatingcongestive heart failure (CHF).

Referring once again to FIG. 2, one can see, for example, the bipolarleads 104 a and 104 b that could be routed, for example, to the distalTIP and RING into the right ventricle. The bipolar leads 104 c and 104 dcould be routed to a distal TIP and RING in the right atrium. There isalso an RF telemetry pin antenna 116 which is not connected to the IS-1or DS-1 connector block. This acts as a short stub antenna for pickingup telemetry signals that are transmitted from the outside of the device100.

It should also be obvious to those skilled in the art that all of thedescriptions herein are equally applicable to other types of AIMDs.These include implantable cardioverter defibrillators (ICDs),neurostimulators, including deep brain stimulators, spinal cordstimulators, cochlear implants, incontinence stimulators and the like,and drug pumps. The present invention is also applicable to a widevariety of minimally invasive AIMDs. For example, in certain hospitalcath lab procedures, one can insert an AIMD for temporary use such as anICD. Ventricular assist devices also can fall into this type ofcategory. This list is not meant to be limiting, but is only example ofthe applications of the novel technology currently described herein.

FIG. 3 is an enlarged, fragmented cross-sectional view taken generallyalong line 3-3 of FIG. 2. Here one can see in cross-section the RFtelemetry pin 116 and the bipolar leads 104 a and 104 c which would berouted to the cardiac chambers by connecting these leads to the internalconnectors 118 of the IS-1 header block 112 (FIG. 2). These connectorsare designed to receive the plug 110 which allows the physicians tothread leads through the venous system down into the appropriatechambers of the heart 114. It will be obvious to those skilled in theart that tunneling of deep brain electrodes or neurostimulators areequivalent.

Referring back to FIG. 3, one can see a prior art feedthrough capacitor120 which has been bonded to the hermetic terminal assembly 106. Thesefeedthrough capacitors are well known in the art and are described andillustrated in U.S. Pat. Nos. 5,333,095, 5,751,539, 5,978,204,5,905,627, 5,959,829, 5,973,906, 5,978,204, 6,008,980, 6,159,560,6,275,369, 6,424,234, 6,456,481, 6,473,291, 6,529,103, 6,566,978,6,567,259, 6,643,903, 6,675,779, 6,765,780 and 6,882,248. In this case,a rectangular quadpolar feedthrough capacitor 120 is illustrated whichhas an external metalized termination surface 122. It includes embeddedelectrode plate sets 124 and 126. Electrode plate set 124 is known asthe ground electrode plate set and is terminated at the outside of thecapacitor 120 at the termination surface 122. These ground electrodeplates 124 are electrically and mechanically connected to the ferrule108 of the hermetic terminal assembly 106 using a thermosettingconductive polyimide or equivalent material 128 (equivalent materialswill include solders, brazes, conductive epoxies and the like). In turn,the hermetic seal terminal assembly 106 is designed to have its titaniumferrule 108 laser welded 130 to the overall housing 102 of the AIMD 100.This forms a continuous hermetic seal thereby preventing body fluidsfrom penetrating into and causing damage to the electronics of the AIMD.

It is also essential that the leads 104 and insulator 136 behermetically sealed, such as by the gold brazes or glass seals 132 and134. The gold braze 132 wets from the titanium ferrule 108 to thealumina ceramic insulator 136. In turn, the ceramic alumina insulator136 is also gold brazed at 134 to each of the leads 104. The RFtelemetry pin 116 is also gold brazed at 138 to the alumina ceramicinsulator 136. It will be obvious to those skilled in the art that thereare a variety of other ways of making such a hermetic terminal. Thiswould include glass sealing the leads into the ferrule directly withoutthe need for the gold brazes.

As shown in FIG. 3, the RF telemetry pin 116 has not been included inthe area of the feedthrough capacitor 120. The reason for this is thefeedthrough capacitor 120 is a very broadband single element EMI filterwhich would eliminate the desirable telemetry frequency.

FIG. 4 is a bottom view taken generally along line 4-4 in FIG. 3. Onecan see the gold braze 132 which completely seals the hermetic terminalinsulator 136 into the overall titanium ferrule 108. One can also seethe overlap of the capacitor attachment materials, shown as athermosetting conductive adhesive 128, which makes contact to the goldbraze 132 that forms the hermetic terminal 106.

FIG. 5 is an isometric view of the feedthrough capacitor 120. As one cansee, the termination surface 122 connects to the capacitor's internalground plate set 124. This is best seen in FIG. 6 where ground plate set124, which is typically silk-screened onto ceramic layers, is broughtout and exposed to the termination surface 122. The capacitor's four(quad polar) active electrode plate sets 126 are illustrated in FIG. 7.In FIG. 6 one can see that the leads 104 are in non-electricalcommunication with the ground electrode plate set 124. However, in FIG.7 one can see that each one of the leads 104 is in electrical contactwith its corresponding active electrode plate set 126. The amount ofcapacitance is determined by the overlap of the active electrode platearea 126 over the ground electrode plate area. One can increase theamount of capacitance by increasing the area of the active electrodeplate set 126. One can also increase the capacitance by addingadditional layers. In this particular application, we are only showingsix electrode layers: three ground plates 124 and three active electrodeplate sets 126 (FIG. 3). However, 10, 60 or even more than 100 such setscan be placed in parallel thereby greatly increasing the capacitancevalue. The capacitance value is also related to the dielectric thicknessor spacing between the ground electrode set 124 and the active electrodeset 126. Reducing the dielectric thickness increases the capacitancesignificantly while at the same time reducing its voltage rating. Thisgives the designer many degrees of freedom in selecting the capacitancevalue.

In the following description, functionally equivalent elements shown invarious embodiments will often be referred to utilizing the samereference number.

FIG. 8 is a general diagram of a unipolar active implantable medicaldevice system 100. The housing 102 of the active implantable medicaldevice 100 is typically titanium, ceramic, stainless steel or the like.Inside of the device housing are the AIMD electronic circuits. UsuallyAIMDs include a battery, but that is not always the case. For example,for a Bion, it can receive its energy from an external pulsing magneticfield. A lead 104 is routed from the AIMD 100 to a point 140 where it isembedded in or affixed to body tissue. In the case of a spinal cordstimulator 100H, the distal TIP 140 could be in the spinal cord. In thecase of a deep brain stimulator 100B, the distal electrode 140 would beplaced deep into the brain, etc. In the case of a cardiac pacemaker100C, the distal electrode 140 would typically be placed in the cardiacright ventricle.

FIG. 9 is very similar to FIG. 8 except that it is a bipolar system. Inthis case, the electric circuit return path is between the two distalelectrodes 140 and 140′. In the case of a cardiac pacemaker 100C, thiswould be known as a bipolar lead system with one of the electrodes knownas the distal TIP 142 and the other electrode which would float in theblood pool known as the RING 144 (see FIG. 10). In contrast, theelectrical return path in FIG. 8 is between the distal electrode 140through body tissue to the conductive housing 102 of the implantablemedical device 100.

FIG. 10 illustrates a bipolar lead system with a distal TIP 142 and RING144 typically as used in a cardiac pacemaker 100C. In all of theseapplications, the patient could be exposed to the fields of an MRIscanner or other powerful emitter used during a medical diagnosticprocedure. Currents that are directly induced in the lead system 104 cancause heating by I²R losses in the lead system or by heating caused bycurrent flowing in body tissue. If these currents become excessive, theassociated heating can cause damage or even destructive ablation to bodytissue.

The distal TIP 142 is designed to be implanted into or affixed to theactual myocardial tissue of the heart. The RING 144 is designed to floatin the blood pool. Because the blood is flowing and is thermallyconductive, the RING 144 structure is substantially cooled. In theory,however, if the lead curves, the RING 144 could also touch and becomeencapsulated by body tissue. The distal TIP 142, on the other hand, isalways thermally insulated by surrounding body tissue and can readilyheat up due to the RF pulse currents of an MRI field.

FIG. 11 is a schematic diagram showing a parallel combination of aninductor L and a capacitor C to be placed in the implantable leadsystems 104 previously described. This combination forms a parallel tankcircuit or bandstop filter 146 which will resonate at a particularfrequency (f_(r)).

FIG. 12 gives the frequency of resonance equation f_(r) for the paralleltank circuit 146 of FIG. 11: where f_(r) is the frequency of resonancein hertz, L is the inductance in henries and C is the capacitance infarads. MRI systems vary in static field strength from 0.5 Tesla all theway up to 3 Tesla with newer research machines going much higher. Thisis the force of the main static magnetic field. The frequency of thepulsed RF field associated with MRI is found by multiplying the staticfield in Teslas times 42.45. Accordingly, a 3 Tesla MRI system has apulsed RF field of approximately 128 MHz.

Referring once again to FIG. 12, one can see that if the values of theinductor and the capacitor are selected properly, one could obtain aparallel tank resonant frequency of 128 MHz. For a 1.5 Tesla MRI system,the RF pulse frequency is 64 MHz. Referring to FIG. 12, one can see thecalculations assuming that the inductor value L is equal to onenanohenry. The one nanohenry comes from the fact that given the smallgeometries involved inside of the human body, a very large inductor willnot be possible. This is in addition to the fact that the use of ferritematerials or iron cores for such an inductor are not practical for tworeasons: 1) the static magnetic field from the MRI scanner would alignthe magnetic dipoles (saturate) in such a ferrite and therefore make theinductor ineffective; and 2) the presence of ferrite materials willcause severe MRI image artifacts. What this means is that if one wereimaging the right ventricle of the heart, for example, a fairly largearea of the image would be blacked out or image distorted due to thepresence of these ferrite materials and the way it interacts with theMRI field. It is also important that the inductance value not vary whilein the presence of the main static field.

The relationship between the parallel inductor L and capacitor C is alsovery important. It has been discovered that maximizing the inductancefor the space available will change the L-C ratios and lead to a higherimpedance at resonance. Accordingly, it is preferred to include as muchreasonable inductance as is possible into a small space and then solvefor the capacitance values. For example, a typical bandstop filter mightutilize inductance of 440 nanohenries in parallel with capacitance of15.9 picofarads. In another embodiment, where space is limited, themaximum inductance achievable is 180 nanohenries in a device designed tomeet a 5-amp AED pulse rating. Accordingly, it is a feature of thepresent invention that the values of capacitance and inductance beselected within the range of 0.1-20,000 picofarads and 1-4000nanohenries, respectively, such that the bandstop filter is resonant atthe selected frequency. In a preferred embodiment, the capacitance wouldbe in the range from 1-100 picofarads and the inductance would be in therange from 100-1000 nanohenries.

It should be also noted that below resonance, particularly at very lowfrequencies, the current in the parallel L-C bandstop filter passesthrough the inductor element. Accordingly, it is important that theparasitic resistance of the inductor element be quite low. Conversely,at very low frequencies, no current passes through the capacitorelement. At high frequencies, the reactance of the capacitor elementdrops to a very low value. However, as there is no case where it isactually desirable to have high frequencies pass through the tankfilter, the parasitic resistive loss of the capacitor is notparticularly important. This is also known as the capacitor's equivalentseries resistance (ESR). A component of capacitor ESR is the dissipationfactor of the capacitor (a low frequency phenomena). Off of resonance,it is not particularly important how high the capacitor's dissipationfactor or overall ESR is when used as a component of a parallel tankcircuit 146 as described herein. Accordingly, an air wound inductor isthe ideal choice because it is not affected by MRI signals or fields.Because of the space limitations, however, the inductor will not be veryvolumetrically efficient. For this reason, it is preferable to keep theinductance value relatively low (in the order of 1 to 100 nanohenries).

Referring once again to FIG. 12, one can see the calculations forcapacitance by algebraically solving the resonant frequency f_(r)equation shown for C. Assuming an inductance value of one nanohenry, onecan see that 6 nano-farads of capacitance would be required. Sixnano-farads of capacitance is a relatively high value of capacitance.However, ceramic dielectrics that provide a very high dielectricconstant are well known in the art and are very volumetricallyefficient. They can also be made of biocompatible materials making theman ideal choice for use in the present invention.

FIG. 13 is a graph showing impedance versus frequency for the paralleltank, bandstop filter circuit 146 of FIG. 11. As one can see, usingideal circuit components, the impedance measured between points A and Bfor the parallel tank circuit 146 shown in FIG. 11 is very low (zero)until one approaches the resonant frequency f_(r). At the frequency ofresonance, these ideal components combine together to look like a veryhigh or, ideally, an infinite impedance. The reason for this comes fromthe denominator of the equation Z_(ab) for the impedance for theinductor in parallel with the capacitor shown as FIG. 14. When theinductive reactance is equal to the capacitive reactance, the twoimaginary vectors cancel each other and go to zero. Referring to theequations in FIGS. 14 and 15, one can see in the impedance equation forZ_(ab), that a zero will appear in the denominator when X_(L)=X_(C).This has the effect of making the impedance approach infinity as thedenominator approaches zero. As a practical matter, one does not reallyachieve an infinite impedance. However, tests have shown that severalhundred ohms can be realized which offers a great deal of attenuationand protection to RF pulsed currents from MRI. What this means is thatat one particular unique frequency, the impedance between points A and Bin FIG. 11 will appear very high (analogous to opening a switch).Accordingly, it would be possible, for example, in the case of a cardiacpacemaker, to design the cardiac pacemaker for compatibility with onesingle popular MRI system. For example, in the AIMD patient literatureand physician manual it could be noted that the pacemaker lead systemhas been designed to be compatible with 3 Tesla MRI systems.Accordingly, with this particular device, a distal TIP bandstop filter146 would be incorporated where the L and the C values have beencarefully selected to be resonant at 128 MHz, presenting a high oralmost infinite impedance at the MRI pulse frequency.

FIG. 16 is a schematic drawing of the parallel tank circuit 146 of FIG.11, except in this case the inductor L and the capacitor C are notideal. That is, the capacitor C has its own internal resistance R_(C),which is otherwise known in the industry as dissipation factor orequivalent series resistance (ESR). The inductor L also has a resistanceR_(L). For those that are experienced in passive components, one wouldrealize that the inductor L would also have some parallel capacitance.This parasitic capacitance comes from the capacitance associated withadjacent turns. However, the inductance value contemplated is so lowthat one can assume that at MRI pulse frequencies, the inductor'sparallel capacitance is negligible. One could also state that thecapacitor C also has some internal inductance which would appear inseries. However, the novel capacitors described below are very small orcoaxial and have negligible series inductance. Accordingly, the circuitshown in FIG. 16 is a very good approximation model for the novelparallel tank circuits 146 as described herein.

This is best understood by looking at the FIG. 16 circuit 146 at thefrequency extremes. At very low frequency, the inductor reactanceequation is X_(L)=2πfL (reference FIG. 15). When the frequency f isclose to zero (DC), this means that the inductor looks like a shortcircuit. It is generally the case that biologic signals are lowfrequency, typically between 10 Hz and 1000 Hz. For example, in acardiac pacemaker 100C, all of the frequencies of interest appearbetween 10 Hz and 1000 Hz. At these low frequencies, the inductivereactance X_(L) will be very close to zero ohms. Over this range, on theother hand, the capacitive reactance X_(C) which has the equationX_(C)=1/(2πfc) will look like an infinite or open circuit (referenceFIG. 15). As such, at low frequencies, the impedance between points Aand B in FIG. 16 will equal to R_(L). Accordingly, the resistance of theinductor (R_(L)) should be kept as small as possible to minimizeattenuation of biologic signals or attenuation of stimulation pulses tobody tissues. This will allow biologic signals to pass through thebandstop filter 146 freely. It also indicates that the amount ofcapacitive loss R_(C) is not particularly important. As a matter offact, it would be desirable if that loss were fairly high so as to notfreely pass very high frequency signals (such as undesirable EMI fromcellular phones). It is also desirable to have the Q of the circuitshown in FIG. 16 relatively low so that the bandstop frequency bandwidthcan be a little wider. In other words, in a preferred embodiment, itwould be possible to have a bandstop wide enough to block both 64 MHzand 128 MHz frequencies thereby making the medical device compatible foruse in both 1.5 Tesla and 3 Tesla MRI systems.

FIG. 17 is a drawing of the unipolar AIMD lead system, previously shownin FIG. 8, with the bandstop filter 146 of the present invention addednear the distal electrode 140. As previously described, the presence ofthe tank circuit 146 will present a very high impedance at one or morespecific MRI RF pulse frequencies. This will prevent currents fromcirculating through the distal electrode 140 into body tissue at thisselected frequency(s). This will provide a very high degree of importantprotection to the patient so that overheating does not cause tissuedamage.

FIG. 18 is a representation of the novel bandstop filter 146 usingswitches that open and close at various frequencies to illustrate itsfunction. Inductor L has been replaced with a switch S_(L). When theimpedance of the inductor is quite low, the switch S_(L) will be closed.When the impedance or inductive reactance of the inductor is high, theswitch S_(L) will be shown open. There is a corresponding analogy forthe capacitor element C. When the capacitive reactance looks like a verylow impedance, the capacitor switch S_(C) will be shown closed. When thecapacitive reactance is shown as a very high impedance, the switch S_(C)will be shown open. This analogy is best understood by referring toFIGS. 19, 20 and 21.

FIG. 19 is the low frequency model of the bandstop filter 146. At lowfrequencies, capacitors tend to look like open circuits and inductorstend to look like short circuits. Accordingly, switch S_(L) is closedand switch S_(C) is open. This is an indication that at frequenciesbelow the resonant frequency of the bandstop filter 146 that currentswill flow only through the inductor element and its correspondingresistance R_(L). This is an important consideration for the presentinvention that low frequency biological signals not be attenuated. Forexample, in a cardiac pacemaker, frequencies of interest generally fallbetween 10 Hz and 1000 Hz. Pacemaker pacing pulses fall within thisgeneral frequency range. In addition, the implantable medical device isalso sensing biological frequencies in the same frequency range.Accordingly, such signals must be able to flow readily through thebandstop filter's inductor element. A great deal of attention should bepaid to the inductor design so that it has a very high quality factor(Q) and a very low value of parasitic series resistance R_(L).

FIG. 20 is a model of the novel bandstop filter 146 at its resonantfrequency. By definition, when a parallel tank circuit is at resonance,it presents a very high impedance to the overall circuit. Accordingly,both switches S_(L) and S_(C) are shown open. For example, this is howthe bandstop filter 146 prevents the flow of MRI currents throughpacemaker leads and/or into body tissue at a selected MRI RF pulsedfrequency.

FIG. 21 is a model of the bandstop filter 146 at high frequency. At highfrequencies, inductors tend to look like open circuits. Accordingly,switch S_(L) is shown open. At high frequencies, ideal capacitors tendto look like short circuits, hence switch S_(C) is closed. It should benoted that real capacitors are not ideal and tend to degrade inperformance at high frequency. This is due to the capacitor's equivalentseries inductance and equivalent series resistance. Fortunately, for thepresent invention, it is not important how lossy (resistive) thecapacitor element C gets at high frequency. This will only serve toattenuate unwanted electromagnetic interference from flowing in the leadsystem. Accordingly, in terms of biological signals, the equivalentseries resistance R_(C) and resulting quality factor of the capacitorelement C is not nearly as important as the quality factor of theinductor element L. The equation for inductive reactance (X_(L)) isgiven in FIG. 15. The capacitor reactance equation (X_(C)) is also givenin FIG. 15. As one can see, when one inserts zero or infinity for thefrequency, one derives the fact that at very low frequencies inductorstend to look like short circuits and capacitors tend to look like opencircuits. By inserting a very high frequency into the same equations,one can see that at very high frequency ideal inductors look like aninfinite or open impedance and ideal capacitors look like a very low orshort circuit impedance.

FIG. 22 is a decision tree block diagram that better illustrates thedesign process herein. Block 148 is an initial decision step thedesigner must make. For illustrative purposes, we will start with avalue of inductance that is convenient. This value of inductance isgenerally going to relate to the amount of space available in the AIMDlead system and other factors. These values for practical purposesgenerally range in inductance value from one nanohenry up to about 4000nanohenries. This puts practical boundaries on the amount of inductancethat can be effectively packaged within the scope of the presentinvention. In the preferred embodiment, one will typically select aninductance value generally ranging from 100-1000 nanohenries, and thensolve for a corresponding capacitance value required to be self-resonantat the selected MRI Lamour frequency. Referring back to FIG. 22, onemakes the decision whether the design was L first or C first. If onemakes a decision to assume a capacitance value C first then one isdirected to the left to block 150. In block 150, one does an assessmentof the overall packaging requirements of a distal TIP 142 bandstopfilter 146 and then assumes a realizable capacitance value. So, indecision block 150, we assume a capacitor value. We then solve theresonant tank equation f_(r) from FIG. 12 at block 152 for the requiredvalue of inductance (L). We then look at a number of inductor designs tosee if the inductance value is realizable within the space, parasiticresistance R_(C), and other constraints of the design. If the inductancevalue is realizable, then we go on to block 154 and finalize the design.If the inductance value is not realizable within the physical andpractical constraints, then we need to go back to block 150 and assume anew value of capacitance. One may go around this loop a number of timesuntil one finally comes up with a compatible capacitor and an inductordesign. In some cases, one will not be able to achieve a final designusing this alone. In other words, one may have to use a custom capacitorvalue or design in order to achieve a result that meets all of thedesign criteria. That is, a capacitor design with high enough internallosses R_(C) and an inductor design with low internal loss R_(L) suchthat the bandstop filter 146 has the required quality factor (Q), thatit be small enough in size, that it have sufficient current and highvoltage handling capabilities and the like. In other words, one has toconsider all of the design criteria in going through this decision tree.

In the case where one has gone through the left hand decision treeconsisting of blocks 150, 152 and 154 a number of times and keeps comingup with a “no,” then one has to assume a realizable value of inductanceand go to the right hand decision tree starting at block 156 startingwith a value of inductance between 100 and 1000 nanohenries is apreferred approach. One then assumes a realizable value of inductance(L) with a low enough series resistance for the inductor R_(L) such thatit will work and fit into the design space and guidelines. After oneassumes that value of inductance, one then goes to decision block 158and solves the equation C in FIG. 12 for the required amount ofcapacitance. After one finds the desired amount of capacitance C, onethen determines whether that custom value of capacitance will fit intothe design parameters. If the capacitance value that is determined instep 160 is realizable, then one goes on and finalizes the design.However, if it is not realizable, then one can go back up to step 156,assume a different value of L and go through the decision tree again.This is done over and over until one finds combinations of L and C thatare practical for the overall design.

For purposes of the present invention, it is possible to use seriesdiscrete inductors or parallel discrete capacitors to achieve the sameoverall result. For example, in the case of the inductor element L, itwould be possible to use two, three or even more (n) individual inductorelements in series. The same is true for the capacitor element thatappears in the parallel tank filter 146. By adding or subtractingcapacitors in parallel, we are also able to adjust the total capacitancethat ends up resonating in parallel with the inductance.

It is also possible to use a single inductive component that hassignificant parasitic capacitance between its adjacent turns. A carefuldesigner using multiple turns could create enough parasitic capacitancesuch that the coil becomes self-resonant at a predetermined frequency.In this case, the predetermined frequency would be the MRI pulsedfrequency.

Efficiency of the overall tank circuit 146 is also measured in terms ofa quality factor, Q, although this factor is defined differently thanthe one previously mentioned for discrete capacitors and inductors. Thecircuit Q is typically expressed using the following equation:

$Q = \frac{f_{r}}{\Delta\; f_{3{dB}}}$Where f_(r) is the resonance frequency, and Δf_(3dB) shown as points aand b in FIG. 23, is the bandwidth of the bandstop filter 146. Bandwidthis typically taken as the difference between the two measuredfrequencies, f₁ and f₂, at the 3 dB loss points as measured on aninsertion loss chart, and the resonance frequency is the average betweenf₁ and f₂. As can be seen in this relationship, higher Q values resultin a narrower 3 dB bandwidth.

Material and application parameters must be taken into considerationwhen designing tank filters. Most capacitor dielectric materials age1%-5% in capacitance values per decade of time elapsed, which can resultin a shift of the resonance frequency of upwards of 2.5%. In a high-Qfilter, this could result in a significant and detrimental drop in thebandstop filter performance. A lower-Q filter would minimize the effectsof resonance shift and would allow a wider frequency band through thefilter. However, very low Q filters display lower than desirableattenuation behavior at the desired bandstop frequency (see FIG. 23,curve 162). For this reason, the optimum Q for the bandstop filter ofthe present invention will embody a high Q inductor L and a relativelylow Q capacitor C which will result in a medium Q tank filter as shownin curve 164 of FIG. 23.

Accordingly, the “Q” or quality factor of the tank circuit is veryimportant. As mentioned, it is desirable to have a very low loss circuitat low frequencies such that the biological signals not be undesirablyattenuated. The quality factor not only determines the loss of thefilter, but also affects its 3 dB bandwidth. If one does a plot of thefilter response curve (Bode plot), the 3 dB bandwidth determines howsharply the filter will rise and fall. With reference to curve 166 ofFIG. 23, for a tank that is resonant at 128 MHz, an ideal response wouldbe one that had infinite attenuation at 128 MHz, but had zeroattenuation at low frequencies below 1 KHz. Obviously, this is notpossible given the space limitations and the realities of the parasiticlosses within components. In other words, it is not possible (other thanat cryogenic temperatures) to build an inductor that has zero internalresistance. On the other hand, it is not possible to build a perfect(ideal) capacitor either. Capacitors have internal resistance known asequivalent series resistance and also have small amounts of inductance.Accordingly, the practical realization of a circuit, to accomplish thepurposes of the present invention, is a challenging one.

The performance of the circuit is directly related to the efficiency ofboth the inductor and the capacitor; the less efficient each componentis, the more heat loss that results, and this can be expressed by theaddition of resistor elements to the ideal circuit diagram. The effectof lower Q in the tank circuit is to broaden the resonance peak aboutthe resonance frequency. By deliberately using a low Q capacitor, onecan broaden the resonance such that a high impedance (high attenuation)is presented at multiple MRI RF frequencies, for example 64 MHz and 128MHz.

Referring again to FIG. 23, one can see curve 164 wherein a lowresistive loss high Q inductor has been used in combination with arelatively high ESR low Q capacitor. This has a very desirable effect inthat at very low frequencies, the impedance of the tank circuit 146 isessentially zero ohms (or zero dB loss). This means that biologicfrequencies are not undesirably attenuated. However, one can see thatthe 3 db bandwidth is much larger. This is desirable as it will blockmultiple RF frequencies. As one goes even higher in frequency, curve 164will desirably attenuate other high frequency EMI signals, such as thosefrom cellular telephones, microwave ovens and the like. Accordingly, itis often desirable that very low loss inductors be used in combinationwith relatively high loss (and/or high inductance) capacitors to achievea medium or lower Q bandstop filter. Again referring to FIG. 23, one cansee that if the Q of the overall circuit or of the individual componentsbecomes too low, then we have a serious degradation in the overallattenuation of the bandstop filter at the MRI pulse frequencies.Accordingly, a careful balance between component design and tank circuitQ must be achieved.

Referring once again to FIG. 17, one can also increase the value ofR_(C) by adding a separate discrete component in series with thecapacitor element. For example, one could install a small capacitor chipthat had a very low equivalent series resistance and place it in serieswith a resistor chip. This would be done to deliberately raise the valueof R_(C) in the circuit as shown in FIG. 17. By carefully adjusting thisvalue of R_(C), one could then achieve the ideal curve 164 as shown inFIG. 23.

FIG. 24 is a tracing of an actual patient X-ray. This particular patientrequired both a cardiac pacemaker 100C and an implantable cardioverterdefibrillator 1001. The corresponding implantable lead system 104, asone can see, makes for a very complicated antenna and loop couplingsituation. The reader is referred to the article entitled, “Estimationof Effective Lead Loop Area for Implantable Pulse Generator andImplantable Cardioverter Defibrillators” provided by the AAMI PacemakerEMC Task Force.

Referring again to FIG. 24, one can see that from the pacemaker 100C,there is an electrode in both the right atrium and in the rightventricle. Both these involve a TIP and RING electrode. In the industry,this is known as a dual chamber bipolar lead system. Accordingly, thebandstop filters 146 of the present invention would need to be placed atleast in the distal TIP in the right atrium and the distal TIP in theright ventricle from the cardiac pacemaker. One can also see that theimplantable cardioverter defibrillator (ICD) 100I is implanted directlyinto the right ventricle. Its shocking TIP and perhaps its super venacava (SVC) shock coil would also require a bandstop filters of thepresent invention so that MRI exposure cannot induce excessive currentsinto the associated lead system (S). Modern implantable cardioverterdefibrillators (ICDs) incorporate both pacing and cardioverting (shock)features. Accordingly, it is becoming quite rare for a patient to have alead layout as shown in the X-ray of FIG. 24. However, the number ofelectrodes remain the same. There are also newer combined pacemaker/ICDsystems which include biventricular pacemaking (pacing of the leftventricle). These systems can have as many as 9 to even 12 leads.

FIG. 25 is a line drawing of an actual patient cardiac X-ray of one ofthe newer bi-ventricular lead systems with various types of electrodeTIPS shown. The new bi-ventricular systems are being used to treatcongestive heart failure, and make it possible to implant leads outsideof the left ventricle. This makes for a very efficient pacing system;however, the implantable lead system 104 is quite complex. When a leadsystem 104, such as those described in FIGS. 8, 9, 10 and 11, areexposed to a time varying electromagnetic field, electric currents canbe induced into such lead systems. For the bi-ventricular system,bandstop filters 146 would be required at each of the three distal TIPsand optionally at RING and SVC locations.

FIG. 26 illustrates a single chamber bipolar cardiac pacemaker leadshowing the distal TIP 142 and the distal RING 144 electrodes. This is aspiral wound system where the RING coil 104 is wrapped around the TIPcoil 104′. There are other types of pacemaker lead systems in whichthese two leads lay parallel to one another (known as a bifilar leadsystem).

FIG. 27 is a schematic illustration of the area 27-27 in FIG. 26. In thearea of the distal TIP 142 and RING 144 electrodes, bandstop filters 146and 146′ have been placed in series with each of the respective TIP andRING circuits. Accordingly, at MRI pulsed frequencies, an open circuitwill be presented thereby stopping the flow of undesirable RF current.

Although several embodiments of the invention have been described indetail, for purposes of illustration, various modifications of each maybe made without departing from the spirit and scope of the invention.Accordingly, the invention is not to be limited, except as by theappended claims.

What is claimed is:
 1. An implantable lead wire, comprising: a) a lengthextending from a proximal lead wire end to a distal lead wire portionhaving a distal lead wire end, wherein the proximal lead wire end iselectrically connectable to electronic circuits of an implantablemedical device; b) an electrode electrically connected to the distallead wire portion or to the distal lead wire end, wherein the electrodeis contactable to biological tissue; and c) at least one bandstop filtercomprising a capacitor segment having a capacitor segment first endspaced from a capacitor segment second end and an inductor segmenthaving an inductor segment first end spaced from an inductor segmentsecond end, wherein the capacitor and inductor segment first ends areelectrically connected together at a first connection along the leadwire length and the capacitor and inductor segment second ends areelectrically connected together at a second connection along the leadwire so that the parallel connected capacitor and inductor segments as apermanently passive circuit forming the at least one bandstop filter arephysically and electrically connected in series with the lead wiresomewhere along the length thereof, d) wherein the inductor segment hasan inductance ranging from 1 to 4,000 nanohenries and an inductorsegment series resistance (R_(L)) so that an inductor segment reactanceand the inductor segment series resistance permit passage of biologicalsignals at frequencies of about 10 Hz to about 1 kHz along the lead wirefrom the electrode to the proximal lead wire end, and e) wherein thecapacitor segment has a capacitance ranging from 0.1 to 20,000picofarads and a capacitor segment series resistance (R_(C)) so that acapacitor segment reactance and the capacitor segment series resistancesubstantially act as an open circuit to the same biological signals atfrequencies of about 10 Hz to about 1 kHz that the inductor segmentallows to pass along the lead wire, and f) wherein the capacitance, thecapacitor segment series resistance, the inductance and the inductorsegment series resistance result in the at least one bandstop filterhaving a Q with a 3-dB bandwidth that is on the order of MHzsubstantially centered at an MRI RF pulsed resonant frequency.
 2. Theimplantable lead wire of claim 1, wherein the at least one bandstopfilter is resonant in a range of frequencies that include the MRI RFpulsed frequency.
 3. The implantable lead wire of claim 1, including aplurality of bandstop filters, each bandstop filter being electricallyconnected in series with the lead wire somewhere along the lengththereof, wherein the plurality of bandstop filters resonate at a rangeof different and respective MRI RF pulsed frequencies.
 4. Theimplantable lead wire of claim 1 being configured as at least one of thegroup selected from a cochlear implant, a neurostimulator, a brainstimulator, a cardiac pacemaker, a ventricular assist device, anartificial heart, a drug pump, a bone growth stimulator, a bone fusionstimulator, a urinary incontinence device, a pain relief spinal cordstimulator, an anti-tremor stimulator, a gastric stimulator, animplantable cardioverter defibrillator, a congestive heart failuredevice, a pill camera, a probe, and a catheter.
 5. The implantable leadwire of claim 1, wherein the capacitance of the capacitor segment iswithin the range of 1-100 picofarads and the inductance of the inductorsegment is within the range of 100-1000 nanohenries.
 6. The implantablelead wire of claim 1 wherein the Q of the bandstop filter is resonant at64 MHz and at 128 MHz.
 7. An implantable lead wire, comprising: a) alength extending from a proximal lead wire end to a distal lead wireportion having a distal lead wire end, wherein the proximal lead wireend is electrically connectable to electronic circuits of an implantablemedical device; b) an electrode electrically connected to the distallead wire portion or to the distal lead wire end, wherein the electrodeis contactable to biological tissue; and c) at least two bandstopfilters, each comprising a capacitor segment having a capacitor segmentfirst end spaced from a capacitor segment second end and an inductorsegment having an inductor segment first end spaced from an inductorsegment second end, wherein: i) the capacitor and inductor segment firstends of a first one of the at least two bandstop filters areelectrically connected together at a first connection along the leadwire length and the capacitor and inductor segment second ends of thefirst one of the bandstop filters are electrically connected together ata second connection along the lead wire so that the parallel connectedfirst capacitor and inductor segments as a first permanently passivecircuit are physically and electrically connected in series with thelead wire somewhere along the length thereof, and ii) the capacitor andinductor segment first ends of a second one of the at least two bandstopfilters are electrically connected together at a third connection alongthe lead wire length and the capacitor and inductor segment second endsof the second one of the bandstop filters are electrically connectedtogether at a fourth connection along the lead wire so that the parallelconnected second capacitor and inductor segments as a second permanentlypassive circuit are physically and electrically connected in series withthe lead wire somewhere along the length thereof and spaced from thefirst bandstop filter, d) wherein the inductor segment for each bandstopfilter has an inductance ranging from 1 to 4,000 nanohenries and aninductor segment series resistance (R_(L)) so that an inductor segmentreactance and the inductor segment series resistance permit passage ofbiological signals at frequencies of about 10 Hz to about 1 kHz alongthe lead wire from the electrode to the proximal lead wire end, and e)wherein the capacitor segment for each bandstop filter has a capacitancethat ranges from 0.1 to 20,000 picofarads and a capacitor segment seriesresistance (R_(C)) so that a capacitor segment reactance and thecapacitor segment series resistance substantially act as an open circuitto the same biological signals at frequencies of about 10 Hz to about 1kHz that the inductor segment allows to pass along the lead wire, f)wherein the capacitance, the capacitor segment series resistance, theinductance and the inductor segment series resistance result in each ofthe bandstop filters having a respective Q with a 3-dB bandwidth that ison the order of MHz substantially centered at an MRI RF pulsed resonantfrequency, and g) wherein the first one of the at least two bandstopfilters is resonant at 64 MHz and the second one of the at least twobandstop filters is resonant at 128 MHz.
 8. The implantable lead wire ofclaim 7, wherein a Q_(L) of the inductor segment for each bandstopfilter is relatively maximized and a Q_(C) of the capacitor segment isrelatively minimized to reduce a circuit Q of the respective bandstopfilter.
 9. The implantable lead wire of claim 7 including a plurality ofbandstop filters, each bandstop filter being electrically connected inseries with lead wire somewhere along the length thereof, wherein theplurality of bandstop filters resonate at a range of different andrespective MRI RF pulsed frequencies.
 10. The implantable lead wire ofclaim 7 being configured as at least one of the group selected from acochlear implant, a neurostimulator, a brain stimulator, a cardiacpacemaker, a ventricular assist device, an artificial heart, a drugpump, a bone growth stimulator, a bone fusion stimulator, a urinaryincontinence device, a pain relief spinal cord stimulator, ananti-tremor stimulator, a gastric stimulator, an implantablecardioverter defibrillator, a congestive heart failure device, a pillcamera, a probe, and a catheter.
 11. The implantable lead wire of claim7 wherein the capacitance for the capacitor segment of each bandstopfilter is within the range of 1-100 picofarads and the inductance forthe inductor segment of each bandstop filter is within the range of100-1000 nanohenries.